Method of determining at least one shift parameter of a vehicle transmission

ABSTRACT

The present disclosure relates to a method for determining at least one shift parameter of a vehicle transmission ( 3 ), the vehicle transmission ( 3 ) comprising a first clutching device ( 8   a ) and a first speed ratio ( 9   a ); a second clutching device ( 8   b ) and a second speed ratio ( 9   b ); an input; and an output, wherein the input and the output of the transmission are connectable by the engaging first clutching device ( 8   a ) or the second clutching device ( 8   b ). The method comprises the steps:
         performing a shift by disengaging the first clutching device ( 8   a ) and/or engaging the second clutching device ( 8   b ), wherein the first clutching device ( 8   a ) stops transferring torque through the transmission at a first time point, wherein the second clutching device ( 8   b ) starts transferring torque through the transmission at a second time point, and   determining the shift parameter at the first time point and/or at the second time point.

The present document primarily relates to a method of determining at least one shift parameter of a vehicle transmission, to a transmission controller for controlling a vehicle transmission, and to a vehicle driveline including a transmission and a transmission controller. The present document also relates to a method for performing a shift of a vehicle transmission and a method for monitoring a state of a transmission.

In a vehicle, in particular in an off-highway vehicle such as a wheel loader, a backhoe loader, a forklift truck, a mining vehicle, a tractor or the like, power and/or torque transferred between a power source of the vehicle such as a combustion engine or an electric engine and a vehicle output such as a drive axle or one or more wheels should be transmitted through the driveline in a preferably smooth manner including during a gear shift in order to provide good operability and maneuverability and to guarantee a good vehicle performance.

Generally, the operation of a gear shift depends on a number of factors including the type of the transmission fluid, the load of the vehicle, a road condition, driver acceleration pedal position, age of the transmission, wear of the clutches, etc. Some of these factors are slow varying processes over the transmission lifetime, such as wear; others are fast varying such as oil temperature or road conditions. It would be expedient to better cope with these varying factors to provide a smooth and continuous power flow through the vehicle driveline during shifting. Furthermore, it would be beneficial if the wear and ageing of the clutches could be indicated or predicted.

It is an object of the presently proposed subject-matter to devise at least one method which at least partially meets the needs mentioned above.

This object is solved by a method according to claim 1, by a transmission controller configured to perform this method, and by a vehicle driveline comprising said transmission controller. Special embodiments are described in the dependent claims and the following description.

Accordingly, a method for determining at least one shift parameter of a vehicle transmission is proposed. The vehicle transmission comprises a first clutching device and a first speed ratio; a second clutching device and a second speed ratio; an input; and an output. The input and the output of the transmission are connectable by the engaging first clutching device or the second clutching device.

The method includes the steps:

-   -   performing a shift by disengaging the first clutching device         and/or engaging the second clutching device, wherein the first         clutching device stops transferring torque through the         transmission at a first time point, wherein the second clutching         device starts transferring torque through the transmission at a         second time point, and     -   determining the shift parameter at the first time point and/or         at the second time point.

By determining the shift parameter as proposed, future shifts can be performed with a higher accuracy. Furthermore, wear and aging of the transmission can be predicted and taken into account.

Optionally, the first clutching device and/or the second clutching device are hydraulic clutching devices, preferably wet plate clutching devices. In this case, the term “pressure” used in the following disclosure may refer to “hydraulic pressure”. In alternative embodiments, the first and second clutching devices are dry clutching devices.

The method can be executed while performing an upshift or a downshift. Usually, the first clutching device is also known as “off-going clutching device” and the second clutching device may be referred to as “on-coming clutching device”. Alternatively, the first clutching device may be referred to as “opening clutching device” and the second clutching device may be referred to as “closing clutching device”. The methods presented in disclosure can be performed when there is only one clutching device (on-coming or closing) in launch shifts (start to drive from standstill) or two clutching devices (first clutching device and second clutching device) or even more than two clutching devices. For instance, the method can also be executed when there are 4 clutching devices (2 opening clutching devices and 2 closing clutching devices) in double-swap shifts.

Typically, after the shift has been performed, the second clutching device is engaged and transfers all the torque through the transmission while the first clutching device is disengaged. Thus, when a further shift is performed, the second clutching device being engaged will become the “off-going clutching device” corresponding to the first clutching device according to the nomenclature disclosed in this disclosure. Hence, for the next shift the first and second clutching devices will become second and first clutching devices, respectively.

Typically, a shift can be divided into several phases:

-   -   a prefilling phase, during which at least one or all possible         on-coming clutches are prepared before the gear request;     -   a filling phase, during which the piston of the on-coming clutch         is positioned based on an optimized trajectory;     -   an overlap phase, during which torque is transferred from the         off-going clutch to the on-coming clutch:     -   a synchronization phase, during which the slip of the on-coming         clutch is removed: and/or     -   a lock phase, during which the on-coming clutch is brought in a         robust way to full pressure.

By using the methods of the present disclosure, the end point of the overlap phase and/or the beginning of the synchronization phase may be accurately determined. The end point of the overlap phase or the beginning of the synchronization phase can equal the first time point and/or the second time point. In some embodiments, the first time point equals the second time point. This means that as soon as the second clutching device starts transferring torque from the input of the transmission to the output of the transmission, the first clutching device stops transferring torque from the input of the transmission to the output of the transmission. Alternatively, the first time point may be earlier than the second time point. In these embodiments, there is a torque drop during which no torque is transferred through the transmission. For instance, the second clutching device transfers all the torque from the input to the output of the transmission at the second time point if the second time point is equal to or later than the first time point. In a third case, the second time point is earlier than the first time point. In the third case the first clutching device remains closed during the torque transfer, a too high (target) pressure in the second clutching device will cause negative torque through the first clutching device, an internal torque loop in the transmission and a drop in the output torque.

In some embodiments, the method includes one or more of the following steps:

-   -   determining a rotational speed of a second input of the second         clutching device,     -   determining a rotational speed of a second output of the second         clutching device,     -   determining a second slip speed of the second clutching device         while performing the shift, wherein the second slip speed is a         difference between the rotational speed of the second input and         the second output,     -   determining whether the second slip speed falls below or exceeds         a second predetermined threshold, the second time point being         the time point when the second slip speed below or exceeds a         second predetermined threshold, determining the shift parameter         when the second slip speed falls below or exceeds the second         predetermined threshold.

The method can further include one or more of the following steps:

-   -   determining a rotational speed of a first input of the first         clutching device,     -   determining a rotational speed of a first output of the first         clutching device,     -   determining a first slip speed of the first clutching device         while performing the shift, wherein the first slip speed is a         difference between the rotational speed of the first input and         the first output,     -   determining whether the first slip speed falls below or exceeds         a predetermined threshold, the first time point being the time         point when the first slip speed falls below or exceeds a first         predetermined threshold,     -   determining the shift parameter when the first slip speed falls         below or exceeds the first predetermined threshold.

The proposed method can be solely speed-based, e.g. exclusively based on speed information of the inputs and outputs mentioned. The inventors recognized that as the pressure inside the clutching devices is dependent on a number of factors, it is more accurate to determine the shift parameter at the time point when the slip speed exceeds or falls below a predetermined threshold.

Optionally, the rotational speed of the first input is determined based on a rotational speed of the input of the transmission. Furthermore, the rotational speed of the second input may be determined based on a rotational speed of the input of the transmission.

Optionally, the rotational speed of the second output is determined based on a speed of the vehicle and the second speed ratio. Alternatively or additionally, the rotational speed of the second output is determined based on a speed of the engine and the second speed ratio. In this way, no additional speed sensors are required. In a further implementation manner, the rotational speed of the second output may be determined by a second speed sensor arranged at the second output.

Optionally, the rotational speed of the first output is determined based on a speed of the vehicle and the first speed ratio. Alternatively or additionally, the rotational speed of the first output is determined based on a speed of the engine and the first speed ratio. Furthermore, the rotational speed of the first output may be determined by a first speed sensor arranged at the first output.

In addition to the speed information, several other parameters can be used or determined, such as torque or pressure inside at least one of the clutching devices or current or voltage used to actuate the clutching devices. Optionally, the shift parameter may be a first friction coefficient of the first clutching device and/or the shift parameter may be a second friction coefficient of the second clutching device.

The shift parameter may be dependent on the amount of torque at the input of the transmission determined at the corresponding time point. Optionally, an amount of torque at the input of the vehicle transmission is determined at the first time point and/or the second time point. The amount of torque may be estimated based on the rotational speed of the input of the transmission and/or the speed of the vehicle and/or the speed of the engine. Usually, an engine can be characterized by its torque map representing the torque as a function of engine speed and/or accelerator pedal position. Therefore, the amount of torque may also be determined by using a torque map of the engine. As the engine speed and/or the accelerator pedal position are usually continuously measured, no additional sensors may be required for the determination of the amount of torque. In case a torque converter is arranged between the input of the transmission and the engine, the torque may be determined based on a torque converter speed ratio.

The shift parameter may be dependent on the amount of pressure in the corresponding clutching device determined at the corresponding time point. A pressure inside the first clutching device and/or a pressure inside the second clutching device may be determined at the first time point and/or the second time point. For instance, a hydraulic pressure inside the first clutching device and/or a hydraulic pressure inside the second clutching device are/is determined at the first time point and/or the second time point.

The pressure inside the corresponding clutching device may be determined by measuring the pressure in the corresponding clutching device by a pressure sensor. Furthermore, the pressure inside the corresponding clutching device may be determined by using a pressure set point of the corresponding clutching device. The pressure set point may be taken from a pressure profile that is used to pressurize the corresponding clutching device. The pressure set point may be available from a clutch controller.

The pressure inside the first clutching device at the first time point may be referred to as relief pressure, while the pressure inside the second clutching device at the second time point may be denoted as engagement pressure. The pressure inside one of the clutching devices required to transfer the torque through the transmission typically depends on the actual amount of torque to be transferred. For instance, when the amount of torque to be transferred is comparatively high, then the pressure needed to engage the second clutching device will be comparatively high. For instance, when the amount of torque to be transferred is comparatively low, then the pressure needed to engage the second clutching device will be relatively low. On the other hand, when the amount of torque to be transferred is comparatively high, then the pressure needed to engage the second clutching device will be relatively high.

One of the advantages of the present disclosure is that a correct relationship between torque and pressure of the corresponding clutching devices can be found during the overlap phase or at the end of the overlap phase to achieve the unloading of the first clutching device (opening clutch) and loading of the second clutching device (closing clutch) avoiding the torque loss (low target pressure) and internal loop (brake by clutch for high target pressure).

In some embodiments, the shift parameter is a first pressure-torque relationship determined at the first time point. Alternatively or additionally, the shift parameter may be a second pressure-torque relationship determined at the second time point. The corresponding pressure-torque relationship typically comprises the pressure and the torque determined at the corresponding time point. The first pressure-torque relationship may be stored in a first look up table. The second pressure-torque relationship may be stored in a second look-up table. For instance, the pressure-torque relationship of the first clutching device is stored in a first relief pressure-torque look-up table and the pressure-torque relationship of the second clutching device is stored in a second engagement pressure-torque look-up table. Preferably, the first look-up table and/or the second look-up table are/is updated after each shift that has been performed. The first pressure-torque relationship and the second pressure-torque relationship obtained may be used when performing a future shift. In this way, the pressure-torque relationships are obtained via a learning algorithm that copes with a changing behavior of the transmission over time.

It is noted that the exact amount of pressure inside the clutching devices is not necessarily required for carrying out the methods presented in this disclosure. For instance, the pressure set point of the corresponding clutching device may deviate from a real or actual pressure inside the corresponding clutching device at the corresponding time point. The method can also be carried out if “wrong pressures values” are used or determined at the corresponding time points. For instance, the pressure set point may deviate more than 10% or more than 20% from the actual pressure inside the corresponding clutching device. The pressure-torque relationship can be determined by estimating the pressure set point or using the pressure set point of the pressure profile at the corresponding time point. The system may then use the pressure set point that deviates from the actual pressure inside the corresponding clutching device at the corresponding time point.

Furthermore, the first pressure-torque relationship and/or the second pressure-torque relationship may be used for determining a friction coefficient of the first clutching device and/or the second clutching device, respectively. Thus, the shift parameter may be a first friction coefficient of the first clutching device, wherein the first friction coefficient is based on the pressure-torque relationship and geometry of the first clutching device. The shift parameter may further be a second friction coefficient of the second clutching device, wherein the second friction coefficient is based on the pressure-torque relationship and geometry of the second clutching device.

Typically, as the transmission ages and wears, the friction coefficient of the clutching devices decreases and a higher pressure within the clutching devices is required to engage the clutching devices. By determining the friction coefficient of the clutching devices, it is possible to predict a current state (e.g. wear) of the clutching devices. The friction coefficient of the corresponding clutching device can thus be used to diagnose and predict behavior of the clutching device or the transmission as a whole, see also below. The corresponding friction coefficient μ_(i) may be determined using the following equation:

μ_(i)=τ_(i)/[(p _(i) −p _(i,kiss))R _(i) N _(i) A _(i)]

where τ_(i) is the amount of torque determined at the corresponding time point, p_(i) is the engagement pressure of the corresponding clutching device, p_(i,kiss) is the relief pressure of the corresponding clutching device, R_(i) is a friction plate radius, A_(i) is a friction plate area, and N_(i) is the number of friction plates of the corresponding clutching device. In the equation above, the index i refers to the corresponding clutching device: i=1 for the first clutching device and i=2 for the second clutching device.

The kiss pressure or relief pressure may be taken from a kiss pressure look up table or a relief pressure look up table, which may be updated after each shift that has been performed. In case the clutching devices are wet plate clutching devices, the kiss pressure or relief pressure usually indicates a hydraulic force necessary to counteract a spring force once a piston of the clutching device is placed against the set of friction plates of the clutching device. Usually, the kiss pressure or relief pressure is dependent on the temperature of the hydraulic fluid and may be thus used to take into account the temperature dependence of the clutching devices. The kiss pressure or relief pressure can be determined by estimating the pressure inside the first clutching device at the first time point. The kiss pressure or relief pressure of the second clutching device is usually determined in a previous shift or a future shift and stored in a look up table. Alternatively or additionally, the kiss pressure/relief pressure of one of the clutching devices or both can be determined by using the methods and systems disclosed in document WO 2014/140041 A2, which is incorporated herein by reference in its entirety. In some cases, however, the kiss pressure or relief pressure p_(kiss) may not be determined, as the correction of the kiss pressure in the above equation is typically only about 0-1 bar, e.g. 0.5 bar, whereas the pressure inside the corresponding clutching devices may amount to 10-20 bar or even more. The equation for determining the friction coefficient μ_(i) above may thus be simplified to

μ_(i)=τ_(i)/[p _(i) R _(i) N _(i) A _(i)].

This equation may be further simplified depending on the application and the required precision of the determination of the friction coefficient. In addition to or instead of the parameters mentioned above, other parameters may be used when determining the friction coefficient (e.g. current, voltage or other related parameters, see below). In this case, the corresponding friction coefficient may be based on a geometry of the clutching device and at least one parameter measured during the shift e.g. at the first time point and/or the second time point.

On the other hand, a more precise determination of the pressure inside the first and/or second clutching devices can be made by further compensating the pressure inside the first clutching device and/or the second clutching device by subtracting a mean dynamic pressure P _(c) accounting for a dynamic pressure building up in the first and/or second clutching device due to a rotation of a clutch shaft. In the equations shown above, p_(i) may then refer to P_(i)−P _(c), where P_(i) is a stagnation pressure coming e.g. from a device pressurizing the corresponding clutching device such as a valve. The calculation of the mean dynamic pressure P _(c) is shown below.

Often, the pressure in a clutching device is provided by using an electric actuator. For instance, to pressurize the clutching device a valve or a solenoid is actuated by applying a current/voltage. Thus, the shift parameter may be dependent on the amount of current and/or voltage in the corresponding clutching device determined at the corresponding time point. In the following, the phrase “current/voltage” denotes “current and/or voltage”. As the current/voltage are linked to the pressure inside the clutching devices, the current/voltage can also be determined instead of the pressure. In this case, a first current/voltage applied to a first actuator actuating the first clutching device may be determined at the first time point. Furthermore, a second current/voltage applied to a second actuator actuating the second clutching device may be determined at the second time point.

In a further embodiment, the shift parameter is a first current/voltage-torque relationship determined at the first time point. Alternatively or additionally, the shift parameter may be a second current/voltage-torque relationship determined at the second time point. The first current/voltage-torque relationship may be stored in a first look up table. The second current/voltage-torque relationship may be stored in a second look-up table. For instance, the current/voltage-torque relationship of the first clutching device is stored in a first relief current/voltage-torque look-up table and the current/voltage-torque relationship of the second clutching device is stored in a second engagement current/voltage-torque look-up table. Preferably, the first look-up table and/or the second look-up table are/is updated after each shift that has been performed. The first current/voltage-torque relationship and the second current/voltage-torque relationship obtained may be used when performing a future shift. In this way, the current/voltage-torque relationships are obtained via a learning algorithm that copes with a changing behavior of the transmission over time.

Furthermore, the first current/voltage-torque relationship and/or the second current/voltage-torque relationship may be used for determining a friction coefficient of the first clutching device and/or the second clutching device, respectively. Thus, the shift parameter may be a first friction coefficient of the first clutching device, wherein the first friction coefficient is based on the current/voltage-torque relationship and geometry of the first clutching device. The shift parameter may further be a second friction coefficient of the second clutching device, wherein the second friction coefficient is based on the current/voltage-torque relationship and geometry of the second clutching device.

The first time point and the second time point can be determined relative to each other. In case the difference between the first time point and the second time point deviates more than a predetermined time interval, a first pressure-profile used to depressurize the first clutching device and/or a second pressure-profile used to pressurize the second clutching device can be changed to reduce the difference between the first time point and the second time point. For instance, by changing the time to empty the first clutching device, the first time point will occur at an earlier point in time or at a later point in time. By changing the time to fill the second clutching device, the second time point will occur at an earlier point in time or at a later point in time. In a preferred embodiment, the difference between the first time point and the second time point kept zero or close to zero, e.g. less than 500 ms or less than 200 ms or less than 100 ms, or less than 50 ms or less than 20 ms.

Moreover, a method for performing a shift of a vehicle transmission is proposed. The method comprises the following steps:

-   -   performing a shift by disengaging the first clutching device         and/or engaging the second clutching device,     -   determining, when performing the shift, torque and the pressure         inside the second clutching device,     -   increasing the pressure inside the second clutching device to a         second pressure-level at which the second clutching device         starts transferring torque through the transmission.

The second pressure-level is based on the pressure-torque relationship of the second look-up table determined in a previous shift according to the method outlined above. In particular, a regularly updated look up table can take into account the ageing of the transmission.

Further, the second clutching device may be pressurized, e.g. filled, using a second pressure profile which may be predetermined. In some examples, the second pressure profile may comprise at least two different pressure values (levels) at different times, e.g. 2, 3, 4 or 5 different pressure values at different times. The second pressure profile may include the second pressure-level. Until the pressure inside the second clutching device has reached the second pressure-level, the pressure inside the second clutching device can be increased by executing a feedforward control procedure. This phase may be called the overlap phase. After the pressure inside the second clutching device has been increased to the second pressure-level, the pressure inside the second clutching device can be further increased by executing a feedback control procedure using the pressure as control variable, a second slip speed as a process variable and a reference slip speed profile as a process variable. The reference slip speed profile may be based on the second friction coefficient and/or the pressure-torque relationship of the second look-up table. In this way, the reference slip speed profile can take the wear and ageing of the clutching device into account. The reference slip speed may be determined by executing a feedforward control procedure. This phase may be referred to as the synchronization phase. If the second slip speed is zero, the rotational speeds of the input and the output of the second clutching device are fully synchronized. In this case, an open loop control procedure may be executed to increase the pressure inside the second clutching device to a predetermined pressure level, which may be e.g. at least 10 bar, 15 bar or 20 bar. This phase may be referred to as the lock phase.

Alternatively or additionally, the method for performing a shift of a vehicle transmission may comprise the following steps:

-   -   performing a shift by disengaging the first clutching device         and/or engaging the second clutching device,     -   determining, when performing the shift, torque and the         engagement pressure inside the first clutching device,     -   decreasing the pressure inside the first clutching device to a         first pressure-level at which the first clutching device stops         transferring torque through the transmission.

The first pressure-level is based on the pressure-torque relationship of the first look-up table determined in a previous shift according to the method outlined above. In particular, a regularly updated look up table can take into account the ageing of the transmission.

Typically, the first clutching device is depressurized, e.g. emptied, using a first pressure profile which may be predetermined. In some examples, the first pressure profile may comprise at least two different pressure values (levels) at different times, e.g. 2, 3, 4 or 5 different pressure values at different times. The first pressure profile may include the first pressure-level. Until the pressure inside the first clutching device has reached the first pressure-level, the pressure inside the first clutching device can be decreased by executing a feedforward control procedure or a feedback control procedure.

According to the above, the at least one shift parameter may include e.g. the first friction coefficient, the second friction coefficient, the first pressure-torque relationship, the second pressure-torque relationship, the first current/voltage-torque relationship, the second current/voltage-torque relationship, the relief voltage/current, the engagement voltage/current, the relief pressure, and/or the engagement pressure.

Furthermore, a method for monitoring a state of a transmission is proposed. The method includes the steps:

-   -   determining the first friction coefficient and/or the second         friction coefficient as described above,     -   comparing the first friction coefficient and/or the second         friction coefficient with a predetermined threshold.

This method is particularly useful for maintenance purposes. For instance, by comparing the friction coefficients with a predetermined threshold it can be estimated when a next service operation or maintenance work of the transmission should be carried out. Furthermore, the method also enables monitoring whether the clutching devices of the transmission are properly adjusted.

The method may further include the step of

-   -   outputting an alert if the first friction coefficient and/or the         second friction coefficient falls below or exceeds the         predetermined threshold.

For instance, if the condition of one of the clutching devices is such that the friction coefficient falls below the predetermined threshold, the clutching device may have to be replaced. Due to the alert the safety of the system may be enhanced. The alert may be output via an alert device. The alert device may include visual indicators such as light-emitting diode (LED) lights or digital screens, audio indicators such as speakers, vibrating elements that allow the alert device to vibrate upon receiving a signal. The alert may be e.g. a vibrating, audible and/or visual alert.

Optionally, the method of shifting the vehicle transmission may include further features as outlined below.

The transmission includes the first clutching device and the second clutching device, wherein at least a portion of the first clutching device may be coupled with or may configured to be coupled with at least a portion of the second clutching device, for example mechanically through one or more shafts, one or more gears, or one or more clutches. The method may further include the steps:

-   -   engaging the second clutching device, wherein engaging the         second clutching device includes controlling a state of the         second clutching device by changing a state of the first         clutching device.

Making use of the coupling between the first and the second clutching device for controlling a state of the second clutching device by way of changing and/or controlling a state of the first clutching device opens up new pathways for controlling the shifting process. This may result in a faster, smoother and more efficient shifting process. The state of the first clutching device that is changed as part of the control procedure aimed at controlling the state of the second clutching device may include an engagement pressure in the first clutching device or a speed of an input shaft of the first clutching device, for example. The state of the second clutching device that is controlled by changing the state of the first clutching device may include a slip speed of the second clutching device, an engagement pressure in the second clutching device or a clutch shaft speed of the second clutching device, for example.

The method may further include disengaging the first clutching device. That is, usually the first clutching device is transferred from an engaged state to a disengaged state during the shifting procedure, while the second clutching device is transferred from a disengaged state to an engaged state.

While the state of the second clutching device is controlled by changing the state of the first clutching device, no torque and/or no power may be transferred through the second clutching device.

Changing the state of the first clutching device may include changing an engagement pressure of the first clutching device, in particular lowering an engagement pressure of the first clutching device. Additionally or alternatively, changing the state of the first clutching device may include changing an input speed and/or an output speed of the first clutching device.

Controlling the state of the second clutching device by changing a state of the first clutching device may include executing a first feedback control procedure. The first feedback control procedure may comprise using an engagement pressure and/or an input speed of the first clutching device as the first control variable. Additionally or alternatively, the first feedback control procedure may comprise using a slip speed and/or an engagement pressure of the second clutching device as the first process variable. The slip speed is the difference between the rotational speed of the clutch input and the clutch output: n_(slip)=s_(in)−n_(out). Thus, when absolute speed values are used a positive slip speed indicates that torque is being transferred or, upon engaging the clutching device, may be transferred from the clutch input to the clutch output. A negative slip speed indicates that torque is being transferred or, upon engaging the clutching device, may be transferred from the clutch output to the clutch input.

The method may further include the steps:

-   -   determining a first torque transmission direction between a         power source of the vehicle and the transmission of the vehicle,         wherein the first torque transmission direction is one of (i)         torque transmission towards the power source and (ii) torque         transmission towards a vehicle output;     -   determining a second torque transmission direction between an         input and an output of the second clutching device based on a         current state of the second clutching device, wherein the input         of the second clutching device is coupled or selectively coupled         with the power source and wherein the output of the second         clutching device is drivingly engaged with or selectively         drivingly engaged with the vehicle output, wherein the second         torque transmission direction is one of (i) torque transmission         towards the power source and (ii) torque transmission towards         the vehicle output;     -   comparing the first torque transmission direction and the second         torque transmission direction; and     -   controlling the state of the second clutching device based on         the outcome of the comparison.

For example, the step of controlling a state of the second clutching device by changing a state of the first clutching device may be carried out only if the first torque transmission direction and the second torque transmission direction are found to be equal.

Determining the first torque transmission direction may include determining a speed ratio between a turbine portion and an impeller portion of a torque converter coupling or selectively coupling the power source with the transmission. Additionally or alternatively, determining the first torque transmission direction may include determining a magnitude and/or a direction of a torsional deflection of a shaft coupling the power source with the transmission.

Similarly, determining the second torque transmission direction may include determining a current speed and/or a rotational direction of the input of the second clutching device and determining a current speed and/or a rotational direction of the output of the second clutching device.

If the outcome of the above-described comparison is that the first torque transmission direction and the second torque transmission direction oppose one another, the step of controlling the state of the second clutching device by changing a state of the first clutching device may include reversing the second torque transmission direction, for example by reversing a sign of a slip speed of the second clutching device.

The method may further include the step of increasing an engagement pressure of the second clutching device by executing a first feedforward control procedure using a first feedforward engagement pressure profile for the second clutching device. The first feedforward profile may include an adaptive end point. The first feedforward profile may be determined based on a current input torque of the second clutching device.

The method may further include decreasing an engagement pressure of the first clutching device by executing at least one of:

-   -   a second feedback control procedure using a slip speed of the         first clutching device as the second process variable and using         the engagement pressure of the first clutching device as the         second control variable; and     -   a second feedforward control procedure using a second         feedforward engagement pressure profile for the first clutching         device.

Decreasing the engagement pressure of the first clutching device and/or the increasing the engagement pressure of the second clutching device may be performed if or once the first torque transmission direction equals the second torque transmission direction.

The above-mentioned feedforward pressure profile/s is/may be reduced by a mean dynamic pressure accounting for a dynamic pressure building up in the first and/or the second clutching device due to a rotation of the clutch shaft, wherein the mean dynamic pressure P_(c) is preferably computed according to:

P _(c)=¼ρω²(R _(o) ² +R _(i) ²−2R _(p) ²),

where ω is an angular speed of the clutch shaft, R_(i), R_(o), R_(p) are local, inner, outer and oil-line pipe radii of the clutch piston, and p is the density of the oil (automatic transmission oil) inside the transmission.

Furthermore, a transmission controller for controlling a vehicle transmission is presently proposed, wherein the transmission controller is configured to carry out at least one of the above-described methods.

The presently proposed vehicle driveline, comprises:

-   -   a power source;     -   a transmission drivingly engaged or selectively drivingly         engaged with the power source;

a vehicle output drivingly engaged or selectively drivingly engaged with the transmission; and

-   -   the above-mentioned transmission controller.

The transmission includes a first clutching device having a first input and a first output, and a second clutching device having a second input and a second output, wherein the first input is drivingly engaged with the second input and wherein the first output is drivingly engaged with the second output.

The driveline may further include a fluid coupling device, in particular a torque converter, the fluid coupling device coupling or selectively coupling the power source with the transmission, in particular with the first input and the second input.

The above as well as other advantages of the presently proposed subject-matter will become readily apparent to those skilled in the art from the following detailed description when considered in the light of the accompanying drawings in which:

FIG. 1 shows an embodiment of the presently proposed vehicle driveline, wherein the driveline includes a transmission and a controller for controlling the transmission;

FIG. 2 shows a flow diagram illustrating different phases of an embodiment of a method of shifting the transmission of FIG. 1, the shifting procedure including handing over a transmission of torque from an off-going clutch to an on-coming clutch;

FIG. 3 shows embodiments of pressure profiles in an off-going clutch and in an on-coming clutch during the shifting procedure of FIG. 2;

FIG. 4 shows embodiments of pressure profiles in the off-going clutch during a pressure drop phase;

FIG. 5 shows embodiments of pressure profiles in the on-coming clutch during a fill phase;

FIG. 6 shows embodiments of pressure profiles in the on-coming clutch during an overlap phase;

FIG. 7 shows embodiments of pressure profiles in the off-going clutch during a pressure release phase;

FIG. 8 shows embodiments of pressure profiles in the on-coming clutch during a lock phase;

FIG. 9 shows a further flow diagram illustrating different phases of an embodiment of a method of shifting the transmission of FIG. 1, the shifting procedure including handing over a transmission of torque from an off-going clutch to an on-coming clutch;

FIG. 10 shows pressure profiles and slip speeds of an off-going clutch and an on-coming clutch during an overlap phase;

FIG. 11 shows pressure profiles and slip speeds of the off-going clutch and the on-coming clutch as well as output speed, turbine speed, drum speed and engine speed during an overlap phase and a synchronization phase; and

FIG. 12 shows a graphical representation of a pressure-torque look up table.

FIG. 1 shows an embodiment of a driveline 1. The driveline 1 comprises a power source 2 such as a combustion engine or an electric engine, a transmission 3, a torque converter 4, a vehicle output or output inertia 5, and a transmission controller 6 in communication with the transmission 3 for controlling the transmission 3. The transmission 3 is coupled or selectively coupled to the power source 2 through the torque converter 4. The torque converter 4 comprises an impeller portion 4 a coupled to the power source 2 and a turbine portion 4 b coupled to the transmission 3. Typically, the torque converter 4 further includes a stator portion (not shown). A lockup clutch 4 c is configured to selectively lock the impeller portion 4 a to the turbine portion 4 b of the torque converter 4, for example to avoid or reduce losses. The transmission 3 is furthermore drivingly engaged or selectively drivingly engaged with the vehicle output 5. The vehicle output 5 may include at least one of a drive shaft, a drive axle, a differential, a final drive, and one or more wheels, for example.

The transmission 3 comprises a first transmission stage 3 a and a second transmission stage 3 b coupled to one another through a drum shaft 7. The first transmission stage 3 a comprises clutching devices 8 a-c, and speed ratios 9 a-c, respectively, wherein the clutching devices 8 a-c and the speed ratios 9 a-c are arranged in parallel between the turbine 4 b and the drum shaft 7 to provide selective driving engagement between the turbine 4 b and the drum shaft 7, respectively. Similarly, the second transmission stage 3 b comprises clutching devices 10 a-c, and speed ratios 11 a-c, respectively, wherein the clutching devices 10 a-c and the speed ratios 11 a-c are arranged in parallel between the drum shaft 7 and the vehicle output 5 to provide selective driving engagement between the drum shaft 7 and the vehicle output 5, respectively. It is understood that the speed ratios 9 a-c may provide different drive ratios and/or different relative directions of rotation between the turbine 4 b and the drum shaft 7. Similarly, it is understood that the speed ratios 11 a-c may provide different drive ratios and/or different relative directions of rotation between the drum shaft 7 and the vehicle output 5.

Each of the clutching devices 8 a-c, 10 a-c have an input facing the power source 2, and an output facing the vehicle output 5. The inputs of the clutching devices 8 a-c of the first transmission stage 3 a are mechanically coupled to one another, as indicated at 12. That is, rotation of one of the inputs of the clutching devices 8 a-c causes rotation of the other inputs of the clutching devices 8 a-c. The outputs of the clutching devices 8 a-c of the first transmission stage 3 a are mechanically coupled to one another, as indicated at 13. That is, rotation of one of the outputs of the clutching devices 8 a-c causes rotation of the other outputs of the clutching devices 8 a-c. The inputs of the clutching devices 10 a-c of the second transmission stage 3 b are mechanically coupled to one another, as indicated at 14. That is, rotation of one of the inputs of the clutching devices 10 a-c causes rotation of the other inputs of the clutching devices 10 a-c. The outputs of the clutching devices 10 a-c of the second transmission stage 3 b are mechanically coupled to one another, as indicated at 15. That is, rotation of one of the outputs of the clutching devices 10 a-c causes rotation of the other outputs of the clutching devices 10 a-c.

The driveline 1 may further include one or more speed sensors (not shown) in communication with the controller 6, for example for measuring some or all of: a speed and/or a direction of rotation of the impeller portion 4 a, of the turbine portion 4 b, the inputs/outputs of the clutching devices 8 a-c, 10 a-c and of the drum shaft 7.

It is understood that the driveline 1 depicted in FIG. 1 may have only one transmission stage or three or more transmission stages. It is furthermore understood that the transmission stages of the driveline 1 may have fewer or more than three clutching devices and drive ratios. However, the driveline 1 includes at least one transmission stage having at least two clutching devices arranged in parallel and coupled to one another.

Some or all of the clutching devices 8 a-c, 10 a-c may be configured as hydraulic clutches such as wet plate clutches which may be actuated by varying a hydraulic pressure in a clutch chamber. Hydraulic clutches are well known in the art of automotive transmissions. Also, some or all of the clutching devices 8 a-c, 10 a-c may be configured as electronically actuatable clutches. Electronically actuatable clutches are likewise well known in the art of automotive transmissions. In both cases, the input and the output of a clutching device may be engaged or locked by increasing an engagement pressure inside the clutching device, for example above a first threshold pressure. Conversely, the input and the output of a clutching device may be disengaged by decreasing or lowering the engagement pressure inside the clutching device, for example below a second threshold pressure.

Torque and/or power may be transmitted between the power source 2 and the vehicle output 5 by closing one clutching device in each of the transmission stages 3 a, 3 b. A gear shift typically includes handing over torque transmission between two clutching devices in at least one of the transmission stages 3 a, 3 b, respectively. For example, shifting the transmission 3 may include transferring the clutching device 8 a in the first transmission stage 3 a from an engaged state to a disengaged state, and transferring the clutching device 8 b in the same transmission stage 3 a from a disengaged state to an engaged state, wherein the engagement and the disengagement process typically overlap in time, for example at least partially.

Each combination of two closed clutching devices (one gear set) has a different speed ratio and by closing the corresponding clutching devices or clutches, this speed ratio is selected.

This presently proposed method relates to the actuation pressure profiles used or employed to operate the clutches 8 a-c, 10 a-c in the transmission 3 to perform a shift or a power shift between two clutches in the same stage 3 a, 3 b, and, more precisely, may furthermore relate to the problem of how to cope with the inherent variability which may occur during such a gearshift.

A shift or a power shift may usually be decomposed into at least three successive phases. For an upshift: pressurizing (e. g. filling) the clutch to be engaged (also denoted on-coming clutch), a torque phase, and an inertia phase. For a downshift, the two latter phases will typically be inverted, and after the pressurizing phase (or fill phase in the case of a hydraulic clutch) there will usually be an additional phase termed torque direction phase which aims at making the direction of the torque flowing into the transmission 3 and the direction of the torque flowing out of the transmission 3 equal. In other words, the presently proposed method addresses the decision whether the first or the second of the above mentioned sequences is employed during a gear shift based on (an estimation of) the sign of the torque flowing through the transmission.

Although applied here to an off-highway application, the techniques described in this document may be employed for controlling any device making use of actuatable clutches. These clutches may be wet-plate clutches actuated with electro-hydraulic proportional valves or by using a ball ramp with an electric actuator, for example.

Given the layout of the transmission 3 depicted in FIG. 1, there typically are many of possible mechanical paths between the power source 1 and the vehicle output 5, and there may be even more possibilities of shifting from one mechanical transmission path to another.

However, there are some requirements which should preferably be fulfilled for a successful gear shift. For example, the torque flow of torque between the power source 1 and the vehicle output 5 should preferably not be interrupted or the torque flow into and out of the transmission 3 should preferably not change direction during the shift. Another requirement is a high shift quality under the influence of changing external factors. Here, the quality of the shift is related to the number of discontinuities of the driveline output torque (also called jerk), since it relates immediately to the vehicle acceleration. Indeed, from the driver perspective, an acceleration discontinuity or interruption occurring during a shift may be experienced as a low quality shift.

Assume there is a closed mechanical path from the torque converter (also abbreviated as TC) 4 to the vehicle output 5. This means that in each stage 3 a, 3 b of the transmission 3 only one of the clutches 8 a-c, 9 a-c, respectively, is closed. The amount of transferred torque and its direction may typically be described by a set of dynamic equations based on the layout of the transmission 3 and possibly additional specifications. When a gear shift is requested, the direction of the flow of torque transmitted via the sticking off-going clutch is very important, wherein the term “off-going clutch” relates to the clutching device which is closed or engaged at the beginning of the shifting process, for example the clutch 8 a, and which is intended to be opened or disengaged during the course of the shifting process. It directly relates to the direction of the flow of torque transmitted via the TC 4 and only the latter will therefore be used. If pressure is applied to the (slipping) on-coming clutch, for example to the clutch 8 b, the direction of the flow of torque transmitted or to be transmitted via the on-coming clutch 8 b is determined by the speeds and rotational directions of the input and of the output of the on-coming clutch 8 b. If the direction of the flow of torque transmitted or to be transmitted via the on-coming clutch 8 b is equal to the direction of the flow of torque transmitted via the TC 4, pressure may be applied to the on-coming clutch 8 b without any problem. If the directions of the flow of torque transmitted via the TC 4 and transmitted or to be transmitted via the on-coming clutch 8 b are not equal, however, pressurizing the on-coming clutch 8 b may result in an unsmooth shift.

The optimal pressure profile to apply to both the off-going clutch 8 a and the on-coming clutch 8 b may depend on a number of conditions such as: temperature of the lubricant (typically oil), the type of the transmission fluid, the load of the vehicle, a road condition, driver acceleration pedal position, age of the transmission, wear of the clutches, etc. Some of these effects are slow varying processes over the transmission lifetime, such as wear; others are fast varying such as oil temperature or road conditions.

In order to cope with these different conditions, the presently proposed shifting method may include a mix of open-loop, feedforward, feedback, and learning control processes. As the operation of a gear shift may be influenced by a number of factors and/or conditions, it is not trivial to obtain a smooth and fast gear shift in all situations without relying on a complex controller logic. Although shifts are variable, it is preferred to implement a generic shift strategy which works with a large number of shift types or preferably with all shift types. For this purpose, the goal is to implement a generic strategy which is preferably applicable to all possible shifts.

The presently proposed method provides a solution to the problem of executing a gear shift in a transmission under varying conditions. The embodiment of the presently propose method described herein comprises a procedural logic for both the on-coming clutch 8 b and the off-going clutch 8 a which is executed in parallel. Within this logic flow, several control procedures may be carried out which aid in coping with variable conditions during a shift. These control procedures may include at least one of a feedforward, feedback or open loop control procedure.

For these different shifts to be executed successfully specific requirements should preferably be met. Some shift types, for example, may require the torque direction to change whereas others do not. FIG. 2 shows a flowchart depicting an embodiment of a generic shift strategy in accordance with the presently proposed method. It provides the shift logic for both the on-coming clutch 8 b and the off-going clutch 8 a. Here and in the following recurring features are designated by the same reference signs.

When the controller 6 does not receive a shift request, the logic is in the WAITING STATE 16 and the currently actuated clutches remain pressurized. When a SHIFT REQUEST is received, the controller 6 starts the logic sequence for both the on-coming clutch 8 b and the off-going clutch 8 a start at the same time. In FIG. 2, the OFF-GOING CLUTCH LOGIC is depicted on the left hand side, and the ON-COMING CLUTCH LOGIC is depicted on the right hand side. At the right end of FIG. 2, a TIME bar 17 is depicted indicating the points in time at which the off-going clutch 8 a and the on-coming clutch 8 b are synchronized. Both the off-going clutch 8 a and the on-coming clutch 8 b have to be synchronized before the procedure may proceed to the next stage. The time bar 17 indicates that:

-   -   the controller 6 starts both ON-COMING CLUTCH LOGIC and         OFF-GOING CLUTCH LOGIC start at the same time after a SHIFT         REQUEST;     -   the controller 6 starts both the OVERLAP PHASE 18 b for the         on-coming clutch 8 b and the OVERLAP PHASE 18 a for the         off-going clutch 8 a at the same time; and     -   The controller 6 ends both the OVERLAP PHASE 18 b for the         on-coming clutch 8 b and the OVERLAP PHASE 18 a for the         off-going clutch 8 a at the same time.

The controller 6 starts the on-coming clutch logic with the FILL PHASE 19. During the fill phase 19 the controller 6 pressurizes or fills the on-coming clutch 8 b to place the on-coming clutch in a state in which torque may be transferred through the clutch. The controller 6 causes the on-coming clutch 8 b to be filled with transmission oil with a certain set pressure profile. This is an open loop control. The controller 6 ends the fill pulse with the initiation of the stabilization phase during which the controller 6 holds the pressure of the on-coming clutch 8 b at the kiss pressure, where the term kiss pressure refers to a value of the clutch pressure at which the input and the output of the clutch are barely in contact but no torque may be transferred through the clutch. The controller 6 keeps the pressure at the kiss pressure until the controller ends the torque direction phase of the off-going clutch 8 a (if the controller 6 executes the torque direction phase 20).

The controller 6 starts the off-going clutch logic with the DROP PHASE 21. During the drop phase 21 the controller 6 decreases the pressure of the off-going clutch 8 a from its initial value to a value where the clutch still sticks. Although this is a feed-forward control process, the controller 6 may switch the control logic to a feedback control process if the controller 6 detects that the off-going clutch 8 a is slipping.

When the pressure of the off-going clutch 8 a has dropped, the controller 6 checks whether the direction of the flow of torque transmitted or to be transmitted by the on-coming clutch 8 a matches the direction of the flow of torque transmitted between the power source 1 and the transmission 3. If it does, the controller 6 initiates the OVERLAP PHASE 18 for both the on-coming clutch 8 b and for the off-going clutch 8 a. If it does not, the controller 6 initiates the TORQUE DIRECTION PHASE 20. During this phase, the controller 6 controls the pressure of the off-going clutch 8 a is changed to control the slip of the on-coming clutch 8 b in a feedback control process using the slip speed of the on-coming clutch 8 b as the process variable and using the pressure of the off-going clutch 8 a as the control variable. The controller 6 terminates the torque direction phase 20 when direction of the flow of torque transmitted via the on-coming clutch 8 b (or to be transmitted upon its closing) equals the direction of the flow of torque transmitted via the TC 4 such that the controller 6 may initiate the OVERLAP PHASE 18.

The controller 6 then initiates the OVERLAP PHASE 18 b for the on-coming clutch 8 b and the OVERLAP PHASE 18 a for the off-going clutch 8 a at the same time. During the OVERLAP PHASE 18 the controller 6 causes the off-going clutch 8 a to hand the transmission of torque between the power source 1 and the vehicle output 5 over to the on-coming clutch 8 b. The overlap phase for the on-coming clutch 8 b is a feed-forward control process while the overlap phase for the off-going clutch 8 a is both a feed-forward and feedback control process.

After the overlap phase 18, the controller 6 causes the off-going clutch 8 a to enter the OFF-GOING RELEASE PHASE 22 during which the controller 6 decreases the pressure of the off-going clutch 8 a from its initial value to its minimum value by means of a feed-forward control process. The controller 6 controls the on-coming clutch 8 b to enter the SYNCHRONIZATION PHASE 23. During the SYNCHRONIZATION PHASE the controller 6 synchronizes the on-coming clutch 8 b using a feedback control procedure using the slip speed of the on-coming clutch 8 b as the process variable and the pressure in the on-coming clutch 8 b as the control variable. After the controller 6 terminates the synchronization of the on-coming clutch 6, the controller 6 initiates the LOCK PHASE during which the controller 6 increases the pressure of the on-coming clutch 8 b from its initial value to its maximum value in an open loop control process.

When the controller 6 has decreased the pressure of the off-going clutch 8 a to its minimum pressure and has increased the pressure of the on-coming clutch 8 b to its maximum pressure, the controller 6 determines that the SHIFT has been executed SUCCESSFULLY and returns the logic to the WAITING STATE 16.

In the following the different phases of the method as depicted in FIG. 2 are described in somewhat more detail. Typical pressure profiles in the off-going clutch 8 a and in the on-coming clutch 8 a during the course of the method schematically depicted in FIG. 2 are illustrated in FIG. 3. The beginning and the end of the different phases shown in FIG. 2 are highlighted in FIG. 3.

Drop Phase

During the drop phase 21, the controller 6 decreases the pressure of the off-going clutch 8 a to a value at which the clutch 8 a is just able to transfer the required torque, using a feed-forward profile. The controller 6 may read this pressure value from a pressure-torque table using estimated transferred torque through the torque converter 4. A way to estimate the torque is described further below.

When the controller 6 has executed this control procedure to the end, the clutch plates of the clutch 8 a are typically still touching and not slipping. If the controller 6 detects any slip during the execution of the feed-forward profile, however, the controller 6 initiates a feedback control process which adapts the pressure of the clutch 8 a such that the off-going clutch 8 a remains in a low-slip mode. Embodiments of typical feedforward pressure profiles used by the controller 6 to control the off-going clutch 8 a during the drop phase 21 are depicted in FIG. 4.

Fill Phase

The controller 6 executes the fill phase 19 to effectively fill the on-coming clutch 8 b with transmission oil. At the end of this phase, the clutch plates should be right at their point of contact without being in a position to transfer torque. This point is called the kiss point. The pressure required to hold the clutch 8 b in this position is called the kiss pressure. The amount of time needed to fill the clutch and its feed lines with oil is called the fill time.

The control procedure applied by the controller 6 during the fill phase 19 is an open-loop control, where the above-mentioned kiss pressure and the fill time are the only two variables. These variables are read from a look-up table. Further details may be inferred from the documents U.S. Pat. No. 9,109,645 and US2014277978 which are incorporated herein by reference in their entirety. An embodiment of a typical pressure profile applied by the controller 6 during the fill phase 19 in the on-coming clutch 8 b is depicted in FIG. 5.

Torque Direction Phase

The gear shift process can typically be grouped into two different shift types. The classification usually depends on the states of the system and, more precisely, on the direction of the flow of torque transmitted through the TC 4 and on the direction of the flow of torque transmitted or to be transmitted through the on-coming clutch 8 b.

If the controller 6 determines that the above-mentioned torque flow directions are equal, the controller 6 skips the torque direction phase 20. However, if the controller 6 determines that the above-mentioned torque flow directions are opposed to one another, the controller 6 initiates the torque direction phase 20 in order to change the slip direction of the on-coming clutch 8 b.

As long as this has not yet happened, pressurizing the on-coming clutch 8 b would result in negative torque being applied to the on-coming clutch 8 b. However, due to the coupling 12, 13 between the inputs and the outputs of the off-going clutch 8 a and the on-coming clutch 8 b, the controller 6 may reverse the slip speed of the on-coming clutch 8 b by changing the pressure in the off-going clutch 8 a. As the controller 6 reduces the pressure in the off-going clutch 8 a, the clutch 8 a transfers less torque, resulting in an acceleration of the input of the off-going clutch 8 a. In this manner the controller 6 may reduce the negative slip speed of the on-coming clutch 8 b until the slip speed of the on-coming clutch 8 b is positive.

However, with the pressure in the off-going clutch 8 a reduced, the output torque of the off-going clutch 8 a is likewise reduced. Thus, a balance between the following two goals should preferably be achieved:

-   -   the off-going clutch pressure should be decreased to reverse the         slip of the on-coming clutch as quickly as possible; and     -   the output torque should preferably be maintained to avoid a         torque dip.

The controller 6 may control the off-going clutch 8 b to follow a slip speed reference—a slip speed profile going from the current value to a smaller value with opposite sign—so that a balance between time and output torque can be achieved. To that end, the controller 6 may control the slip speed of the off-going clutch 8 a to reduce the pressure in the off-going clutch 8 a and may later increase it again to follow this trajectory. When the controller 6 has reached this condition (slip speed sign is reversed), applying pressure on the on-coming clutch 8 b results in positive torque. This is the moment when the controller 6 may initiate the overlap phase 18 during which the on-coming clutch 8 b takes over the torque of the off-going clutch 8 a.

Direction of the Flow of Torque Transmitted Via the TC

As mentioned above, the direction of the flow of torque transmitted via the TC 4 typically plays an important role during the shifting process. Different methods can be used to measure or estimate this direction. The method used does not matter as long as it yields a sufficiently accurate estimate of the input torque at the transmission 3. For example, the direction of the flow of torque transmitted via the TC 4 may be measured via one or more sensors, it may be estimated based on engine or torque converter characteristics, or it may be determined based on the dynamic equations describing the system. The method is illustrated here by making use of the torque converter characteristic.

The TC is typically well characterized by the set of equations shown here below. The T_(p2000) (pump torque at pump speed of 2000 rpm) and the TR (torque ratio at which pump torque is increased to get turbine torque), as a function of the speed ratio, are commonly provided characteristics. They can simply be re-used as lookup tables as a function of the SR_TC.

${{SR}_{TC} = \frac{n_{tur}}{n_{e}}};{T_{p} = {{T_{p\; 2000}\left( {SR}_{TC} \right)}*\frac{n_{e}^{2}}{2000^{2}}}};{T_{tur} = {T_{p}*{{TR}\left( {SR}_{TC} \right)}}}$

Direction of the Flow of Torque Transmitted Via the Slipping Clutch

The controller 6 may estimate the torque direction based on the operating point of the TC 4. Generally, the torque direction is positive (from ICE to output) for speed ratios of the torque converter SR_(TC) smaller than 1. This is called the drive situation. For SR_(TC)>1 the torque direction is usually negative and may be termed the brake situation. Some small variations around the switch point (SR_(TC)=1) are possible, so it is usually preferable to get the torque by using the Tp2000 map at the actual SR_(TC) and to extract the sign of the computed turbine torque. As stated above, the controller 6 may determine the torque direction based on the operating point of the TC 4.

When a new gear is selected one can determine the torque direction when this clutch is actuated in accordance with the calculation shown below. This is typically different than the slip speed definition which may also be provided.

n _(slip) =n _(in) −n _(out) ;T _(dir)=sign(n _(in))·sign(n _(slip))

To reverse the torque direction of the on-coming clutch, the sign of the on-coming clutch slip speed will typically have to be reversed before the clutch starts transferring torque, for example by controlling the off-going clutch pressure.

Overlap Phase: On-Coming Clutch

During the overlap phase 18 the controller 6 typically increases the pressure in the on-coming clutch 8B to a level where the clutch may transfer all the torque coming from the torque converter 4. To this end, the controller 6 may use a feedforward profile with an adaptive end point based on the pressure-torque-relationship of the on-coming clutch 8 b.

The on-coming clutch 8 b is generally in slip during this phase. However, it is conceivable that the on-coming clutch 8 b already starts sticking because the slip speed of the on-coming clutch 8 b is small. In this situation, it is preferred that the controller 6 hold the on-coming clutch 8 b closed and increase the pressure rapidly to avoid that the clutch 8 b starts slipping again. The controller 6 may then end the overlap phase 18 and start the synchronization phase 23 and the lock phase 24 for the on-coming clutch 8 b.

However, if the torque direction phase 20 was executed because the sign of slip was previously not correct and has now been corrected by the controller 6, the slip speed will typically be small already. In this case, the described condition may be ignored to proceed to complete the hand over of the torque. Embodiments of typical pressure profiles of the on-coming clutch 8 b are depicted in FIG. 6.

Overlap Phase: Off-Going Clutch

The goal during this phase typically is to reduce the torque transfer through the off-going clutch 8 a. This phase may be entered from two different previous phases:

-   -   from the drop phase 21 if the torque direction was correct; and     -   from the torque direction phase 20 if the torque direction was         previously not correct but has now been corrected.

In the first case, the off-going clutch 8 a is in stick or slip, depending on how it leaves the drop phase. In the second case, the clutch 8 a is necessarily slipping. In both situations the controller 6 controls the off-going slip speed by using a constant slip speed reference, namely the captured off-going slip speed value at the beginning of the overlap phase 18. To be fully generic, the feedback error may be computed as follows:

error=(n _(s,off) −n _(s,off,capt))*sign(T _(tur,capt))*sign(n _(in)),

where n_(s,off) is the off-going slip speed, n_(s,off,capt) is the captured off-going slip speed at the beginning of the overlap, T_(tur,capt) is the captured turbine torque at the beginning of the overlap, and n_(in) is the clutch input shaft speed.

As the controller 6 increases the on-coming pressure to take over the torque, the pressure in the on-coming clutch 8 b will usually affect the slip speed of the off-going clutch 8 a which is feedback-controlled. The pressure in the on-coming clutch 8 b may therefore also affect the pressure in the off-going clutch 8 a. Indeed, as the on-coming clutch 8 b takes over torque, the applied pressure of the off-going clutch 8 a (when in slip) may be too high, thereby tending to synchronize the off-going clutch 8 a again. Therefore, the controller 6 may reduce the pressure in the off-going clutch 8 a to increase the slip speed again. Ideally, all torque has been handed over to the on-coming clutch 8 b at the end of the overlap phase 18.

In the meantime, a feedforward pressure profile may be applied to the off-going clutch aiming at decreasing its pressure from the level it is at at the beginning of the phase to the clutch relief pressure.

A maximum feed-forward pressure profile may also be computed in order to force the off-going pressure to decrease in case it reaches the computed value. Theoretically, at the end of the overlap phase, the maximum pressure of the off-going clutch equals the kiss pressure. However, due to various hysteresis effects in the complete system, a pressure lower than the kiss pressure should be used. This level is called the relief pressure and may be set as a fraction of the actual kiss pressure. This usually allows directly linking the relief pressure to the kiss pressure, as the latter may varying with temperature, for example. The exit condition used to proceed to the next phase usually includes the off-going pressure reaching the relief pressure level.

Synchronization Phase

At the end of the overlap phase 18, the on-coming clutch 8 b usually transfers all the torque produced by the torque converter 4 but is not necessarily synchronized yet. In this phase, the on-coming clutch 8 b may be synchronized by following a smooth reference slip speed trajectory. The tracking of a slip speed reference smoothly going to zero allows performing the transition from slip to stick in a way that may naturally limit mechanical jerks.

Release Phase for the Off-Going Clutch

Usually, the goal during this phase is to bring the pressure of the off-going clutch 8 a to zero and, in some situations, to hand over torque from the off-going clutch 8 a to the on-coming clutch 8 b.

This part typically uses a feed-forward profile that starts at the last pressure value of the overlap controller. The controller 6 may keep the pressure constant for a moment and may then goes to the relief pressure level. The controller 6 may then reduces the pressure further to zero.

In most situations, the last value of the overlap is already the relief pressure. In cases where the on-coming clutch is synchronized in the overlap phase 18, the captured pressure of the off-going clutch 8 a may be higher and torque must still be transferred from the off-going clutch 8 a to the on-coming clutch 8 b. By reducing the pressure in the off-going clutch 8 a in a smooth way, torque may handed over to the on-coming clutch 8 b. Therefore, this profile is preferably made smooth to cope with these cases. FIG. 7 shows embodiments of typical pressure profiles in the off-going clutch 8 a during the pressure release phase.

Lock Phase

With an open-loop control, the controller 6 may increase the pressure of the on-coming clutch 8 b to the maximum pressure, typically 20 bar, locking the clutch for lossless torque transfer. FIG. 8 shows embodiments of typical pressure profiles in the on-coming clutch 8 b during the lock phase.

Dynamic Pressure Compensation

In order to be able to compensate for the dynamic pressure which may build up in the clutch and which may be a function of the transmission shaft speed, clutch geometry and oil-line pressure and diameter, the controller 6 may apply the following strategy.

In all phases and on both the off-going and on-coming clutches there may be a dynamic pressure generated by the rotation of a mass of oil in the clutches. This mass rotation typically creates a force on the clutch piston due centrifugal effects. This dynamic pressure in the clutches may create disturbances during all the phases of the shift as it may create an extra pressure which may disturb the on-coming clutch pressure which may delay the opening of the off-going clutch.

To compensate for the dynamic pressure, the controller 6 may calculate an estimation in real-time during every shift and may reduce the estimated pressure value from the set pressure profile. The estimation may be made using the relation

P _(c)=¼ρω²(R _(o) ² +R _(i) ²−2R _(p) ²),

where ω is angular shaft speed, P _(c) is mean dynamic-pressure on the clutch-piston due to centrifugal force, R_(i), R_(o), R_(p) are local, inner, outer and oil-line pipe radii of the clutch piston and ρ is the transmission oil (automatic transmission fluid) density. There may also be a compensation for pressure variation caused by the oil pump capacity at different engine speeds (the oil pump may be stronger at higher engine speeds). This model can be used in an LU-clutch of a torque convertor as well as in on-coming and off-going clutches to make the controller robust against different shaft speeds and oil density changes. It may furthermore take into account the geometry of the clutch-piston and the oil line diameter.

How to Handle Various Shift Types

During the shifting process the torque direction (of the TC) can change, for example due to changes in road load, application of the throttle, the nature of the shift (kinematic gear ratios). The gear shift strategy should preferably be able to cope with these situations. If we take these effects into consideration, an exhaustive list of all shift types can be made.

Different shift types are defined by two combined words such as “drive drive”. The first word indicates the state of the system at the start of the shift and the second word refers to the state after the shift. “Drive” means a positive torque flow (from engine to output) and “brake” means negative torque flow from output to engine). Note that brake in this sense has no direct relation with applying the brakes of the vehicle. It refers to the contribution of the powertrain to the vehicle.

A total of 10 shift types can be defined:

1. Simple closing (closing 1 clutch without making mechanical path from TC to out).

2. Launch (Closing the clutch which makes a mechanical path from TC to out).

3. Drive Drive Upshift→Upshift with positive torque to output.

4. Drive Drive downshift→‘Kickdown’ for more traction.

5. Brake Brake upshift→driving downhill reducing engine speed and braking torque.

6. Brake Brake downshift→engine braking increase, downhill increase engine brake, prevent engine stall.

7. Brake drive upshift→TC>1 in lower gear and TC<1 in upper gear.

8. Brake drive downshift→TC>1 in upper gear and TC<1 in lower gear (very unlikely).

9. Drive brake downshift→TC<1 in upper gear and TC>1 in lower gear.

10. Drive brake upshift→TC<1 in lower gear and TC>1 in upper gear (very unlikely)

Simple Closing

There is no off-going clutch. The on-coming clutch performs the fill, synchronization and lock phase. The overlap is not needed because there is no off-going clutch. Thus in this situation the synchronization can start directly after the filling. The shift is ended with the lock phase.

Launch

There is no off-going clutch. Thus, these parts can be skipped. Typically, only the workflow for the on-coming clutch is needed. Usually, filling is needed, the overlap is not needed because there is no off-going clutch. Therefore, in this situation the synchronization can start directly after the filling. The shift may terminate with the lock phase.

Drive Drive Upshift

In the drive drive upshift situation, the output torque is positive before and after the shift. To ensure a good shift with no output torque loss, the following strategy may be used:

-   1. Drop off-going clutch pressure but hold it in stick and fill the     on-coming clutch. -   2. Perform the overlap (torque) phase, the on-coming clutch takes     over torque from the off-going clutch. The off-going clutch is still     in stick at the beginning and it could start to slip during the     overlap phase. At the end of the overlap phase, the off-going     typically does not transfer any torque anymore and the pressure is     equal to or lower than the kiss pressure. -   3. Reduce the last part of pressure in off-going to zero and     synchronize the on-coming clutch.

Drive Drive Downshift

A drive/drive downshift is a downshift where the output torque stays positive. This situation is needed when more traction torque is needed.

The challenge with this shift type is that the torque directions are not equal. The direction of the TC is positive, which is usually desired. However the torque direction of the on-coming clutch (lower gear) is negative at the start of the shift. Some effort must be taken to make the torque direction of the on-coming clutch positive.

This shift may include one or more of the following steps:

-   1. Drop pressure in the off-going clutch but hold it in stick and     already fill the on-coming clutch. -   2. Reduce pressure in the off-going clutch and bring the off-going     clutch into slip. Hold the on-coming clutch at kiss pressure.     Control the slip of the on-coming clutch with off-going pressure     changing the torque direction. -   3. Overlap, take over torque from off-going clutch to on-coming     clutch. -   4. Synchronization -   5. Drop pressure in the off-going clutch but hold it in stick and     already fill the on-coming clutch. -   6. Reduce pressure in the off-going clutch and bring the off-going     clutch into slip. Hold the on-coming clutch at kiss pressure.     Control the slipe of the on-coming clutch by controlling the     pressure of the off-going clutch to change the torque direction. -   7. Overlap, take over torque from the off-going clutch to the     on-coming clutch. But here the on-coming clutch usually already     synchronizes during the overlap -   8. Lock the on-coming clutch and reduce the pressure in the     off-going clutch to hand over torque to the closed clutch

Brake Brake Upshift/Downshift

The brake brake upshift may include the same strategy as the drive drive downshift (see above).

The brake brake downshift may include the same strategy as the drive drive upshift (see above).

Brake Drive Upshift/Downshift

In the brake drive upshift situation, the output torque is negative at the beginning of the shift and changes sign at some point during the process. This may correspond to driving downhill using the engine brake and upshifting to start accelerating the vehicle again. The shift process starts as for a brake brake upshift (see above) the difference being that during one of the four steps, the TC torque direction reverses.

-   -   If the reversal happens during step 1, the shift may proceed         exactly as a drive drive upshift.     -   If the reversal happens during step 2, then releasing pressure         in the off-going clutch may not reverse the slip speed sign.         This will typically be detected by the controller because the         pressure in the off-going clutch may reach the kiss pressure         level (or lower). It may then immediately skip to the         synchronization phase (step 4) since the off-going clutch is not         transferring any torque any more.     -   If the reversal happens during step 3, the fact that the slip         speed is controlled to a low level after step 2 may help to         automatically reverse the torque direction of the on-coming         clutch. Usually, this has no negative effect on the output         torque since at the moment it changes sign, it is also         necessarily at a very low level. Depending on the torque level         after the reversal, either the on-coming clutch torque direction         will match very quickly the TC torque direction, or the         on-coming clutch will be automatically synchronized, helping         step 4.     -   If the reversal happens during step 4, it will only make it         faster.

Typically, the brake drive downshift is unlikely occur. It starts as a brake brake downshift (see above). Usually, the only way to produce a torque reversal while the off-going clutch sticks (which is wanted until the synchronization) is either to slow down the turbine shaft speed by pushing the brake pedal or to accelerate the engine by pushing the acceleration pedal. If the off-going clutch starts to slip, there are typically two possibilities to reverse the TC torque direction: either the consequent turbine shaft deceleration toggles the torque converter from brake to drive condition. If the reversal happens during the drop phase, the shift may seamlessly proceed as for a drive drive downshift (see above). If the reversal happens during the overlap, this usually has no negative effect on the output torque since at the moment it changes sign, it is also necessarily at a very low level. If the reversal happens during the synchronization, it will usually only make it faster.

Drive Brake Downshift/Upshift

In the drive brake downshift situation, the output torque is positive at the beginning of the shift and changes sign at some point during the process. This may correspond to driving under traction and downshifting to slow down the vehicle using engine brake.

The shift process may start as for a drive drive downshift (see above), the difference being that during one of the four steps, the TC torque direction reverses. This situation is the dual of the brake drive upshift and the same comments apply, with the only difference being that “drive drive upshift” needs to be replaced with “brake brake downshift” if the reversal happens during step 1 (see above).

Usually, the drive brake upshift is unlikely to occur. The shift may start as a drive drive upshift (see above) and there may be two possibilities to trigger this shift. First, the off-going clutch starts to slip before the synchronization phase (which is unwanted in this case) and the turbine shaft acceleration toggles the torque converter condition from drive to brake. Second, the torque direction of the torque converter is reversed due to the falling engine speed. This usually means that the driver suddenly completely releases the acceleration pedal in the middle of the shift (the off-going clutch does not need to slip, though). In any case, if the reversal happens during the drop phase, the shift may seamlessly proceed as for a brake brake upshift (see above). If the reversal happens during the overlap, it typically has no negative effect on the output torque since at the moment it changes sign, it usually is at a very low level. If the reversal happens during the synchronization, it will typically only make it faster.

The present disclosure is also related to a method for determining at least one shift parameter of the vehicle transmission 3 described above. The method for determining the shift parameter of the vehicle transmission 3 can be advantageously combined with the steps and features related to the method of shifting the vehicle transmission 3 described above. The method will be further elucidated by FIGS. 9-12.

FIG. 9 shows a further flow diagram illustrating different phases of an embodiment of a method of shifting the transmission of FIG. 1, the shifting procedure including handing over a transmission of torque from the off-going clutch 8 a to the on-coming clutch 8 b. The steps and phases illustrated in FIG. 9 resemble the steps and phases depicted in and with regard to FIG. 2 except otherwise indicated. Thus, the overlap phase 180, the overlap phase 180 a, the overlap phase 180 b, the off-going release phase 220, the synchronization phase 230 and the lock phase 240 may include some or all of the features of the overlap phase 18, the overlap phase 18 a, the overlap phase 18 b, the off-going release phase 22, the synchronization phase 23 and the lock phase 24, respectively, unless otherwise stated. In FIG. 9, the beginning of the overlap phase is indicated by “overlap start 160”.

FIG. 10 shows pressure profiles p1, p2 and slip speeds n1, n2 of the off-going clutch 8 a and the on-coming clutch 8 b, respectively, during the overlap phase 180 a, 180 b, the off-going release phase 220, the synchronization phase 230 and the lock phase 240. The pressure profiles p1, p2 and slip speeds n1, n2 are shown for a given/measured torque transferred from the input of the transmission 3 to the output of the transmission 3. In FIG. 10, the beginning of the overlap phase is indicated by t0, whereas the end of the overlap phase is indicated by t1=t2.

The method for determining at least one shift parameter of the vehicle transmission 3 includes at least the following steps. A shift is performed by disengaging the off-going clutching device 8 a and engaging the on-coming clutching device 8 b. The off-going clutching device 8 a stops transferring torque through the transmission 3 at a first time point t1. The on-coming clutching device starts transferring torque through the transmission 3 at a second time point t2. The shift parameter is determined at the first time point t1 and/or the second time point t2. Usually, second time point t2 marks the end of the overlap phase 18 b, 180 b and the beginning of the synchronization phase 23, 230. Further, first time point t1 marks the end over the overlap phase 18 a, 180 a and the beginning of the off-going release phase 22, 220. Often, it is desired that time points t1 and t2 are equal in time (see FIG. 10). In the following, it is assumed that the first time point t1 and the second time point are equal, thus t1=t2. In some embodiments, t1 and t2 may differ from each other. For instance, t1 may be later or earlier in time than t2.

The slip speed n1 of the off-going clutch 8 a and the slip speed n2 of the on-coming clutch 8 b are determined while performing the shift. Herein, the slip speed n1, n2 is the difference between the rotational speeds of the input and the output of the corresponding clutching device 8 a, 8 b according to the above equation n_(i, slip)=n_(i,in)−n_(i,out), where i=1 for the off-going clutching device 8 a and i=2 for the on-coming clutching device 8 b.

For determining the slip speed n1, a rotational speed of a first input of the off-going clutch 8 a and a rotational speed of a first output of the off-going clutch 8 a is measured or determined. For determining the slip speed n2, a rotational speed of a second input of the on-coming clutch 8 b and a rotational speed of a second output of the on-coming clutch 8 b is measured or determined.

The rotational speed of the first and second inputs of the off-going clutch 8 a and the on-coming clutch 8 b may be determined based on a rotational speed n_(tur) of the input of the transmission 3. The rotational speed of the first and second outputs may be based on a speed n_(out) of the vehicle output 5 (see FIG. 1) and the speed ratios 9 a and 9 b, respectively. Alternatively or additionally, the rotational speed of the first and second outputs is determined based on a speed of the engine 2 and the speed ratios 9 a and 9 b. In this way, existing speed sensors can be used for the determination of the rotational speeds mentioned. Optionally, speed sensors (not shown) may be arranged at the first output of the first clutch 8 a and the first output of the second clutch 8 b to measure the rotational speeds of the first output and the second output, respectively.

The first slip speed n1 and the second slip speed n2 are continuously compared to a first predetermined slip speed threshold value and a second predetermined slip speed threshold, respectively. The first time point t1 marks the point in time at which the first slip speed n1 falls below or exceeds the first slip speed threshold value. The second time point t2 marks the point in time at which the second slip speed n2 falls below or exceeds the second slip speed threshold value. Thus, the shift parameter is determined when the first slip speed n1 and/or the second slip speed n2 fall below or exceed the first predetermined threshold and/or the second predetermined threshold. In FIG. 10 the slip speeds n1 and n2 of the off-going clutch 8 a and the on-coming clutch 8 b are shown during the overlap phase 18, 18 a, 18 b, 180, 180 a, 180 b. At the beginning of the overlap phase at t0 the pressure inside the off-going clutch 8 a still exceeds the kiss pressure/the relief pressure of the off-going clutch 8 a. During the overlap phase the pressure inside the off-going clutch 8 a is decreased, whereas the pressure inside the on-coming clutch 8 b is increased. The slip speeds n1 and n2 remain relatively constant during the overlap phase 18 a, 18 b, 180 a, 180 b. However, at the end of the overlap phase 18 a, 18 b, 180 a, 180 b there is a sudden drop in slip speeds n1 and n2 (see FIG. 10). At time point t1=t2, the determined slip speeds n1 and n2 fall below predetermined thresholds. The shift parameter is then determined at this points t1=t2.

Further, a pressure inside the off-going clutching device 8 a and a pressure inside the on-coming clutching device 8 b are determined at t1=t2. In case the clutching devices 8 a, 8 b are hydraulic clutching devices, the term expression pressure refers to hydraulic pressure, i.e. a hydraulic pressure inside the off-going clutching device 8 a and a pressure inside the on-coming clutching device 8 b are determined at t1=t2. For determining the pressure inside clutching devices 8 a and 8 b pressure sensors (not shown) inside the clutching devices 8 a, 8 b may be used. Alternatively, the pressure inside the clutching devices 8 a, 8 b may be determined using e.g. a pressure set point p12, p22 of a pressure profile p1, p2 used by the controller 6 to pressurize the clutching devices 8 a and 8 b (see FIG. 10).

In some embodiments, the shift parameter may be a first or a second pressure-torque relationship for the corresponding clutching devices 8 a, 8 b. For instance, the second pressure-torque relationship of the on-coming clutching device 8 b is the pressure p2 and the torque determined at the time point t1=t2. The pressure-torque relationship is stored in a look-up table. The amount of pressure that is to be applied to the off-going clutching device 8 a (relief pressure) and the on-coming clutching device 8 b (engagement pressure) typically depends on the amount of torque that is to be transferred from the input of the transmission to the output of the transmission. Each time a shift is performed, the look-up table including the pressure-torque relationship of the corresponding clutching device 8 a, 8 b is updated.

FIG. 12 is a graphical representation of a look up table of the on-coming clutching device 8 b. FIG. 12 shows a plurality of engagement pressure-torque relationships of the on-coming clutching device 8 b measured at the time point t1=t2 and after a plurality of consecutive shifts. As can be seen from FIG. 12, a resulting vector 80 has been drawn through the pressure-torque relationships obtained after the plurality of shifts. In FIG. 12 an original vector 82 is also shown depicting a theoretical pressure-torque relationship immediately after manufacture of the transmission 3. As is apparent from FIG. 12, the offset and the slope of the resulting vector 80 are increased compared to the offset and slope of the original vector 82 due to wear and ageing of the transmission 3.

It should be mentioned that the exact amount of pressure inside the clutching devices 8 a, 8 b is not necessarily required for carrying out the method described above. Instead of measuring the pressure inside the clutching devices 8 a, 8 b, a pressure set point obtained from the pressure profile p1, p2 used by the controller 6. The pressure set point p12, p22 may deviate from the actual pressure inside the clutching devices 8 a, 8 b e.g. by more than 25%. For instance, if the set point pressure at the second time point t2 for the on-coming clutching device 8 b is 10 bar, and the actual pressure in the on-coming clutching device 8 b is 4 bar, the controller 6 may update the pressure value in the pressure-torque look up table to 10 bar. The next time a similar shift is performed, the stored pressure value of 10 bar may be used, corresponding to an actual value of 4 bar. Thus, the method may function well even if the pressure stored in the pressure-torque look-up table contains an error.

In FIG. 10, the feedforward pressure profile p2 is estimated based on a “moving target”, that is, the amount of torque T_(tur) is estimated from a speed of the vehicle at the beginning of the overlap phase 18 a, 18 b, 180 a, 180 b and the pressure-torque relationship stored in the second look-up table. Since driving speed, driver acceleration pedal position, and torque are connected to each other via a torque map for each specific engine, the amount of torque T_(tur) can be readily taken from said torque map and the characteristics of the TC (see above). For updating the pressure-torque look-up table, an amount of torque T_(tur) at the output of the turbine is determined at t1=t2.

In some embodiments, the shift parameter is a first friction coefficient μ₁ of the off-going clutch 8 a and/or a second friction coefficient μ₂ of the on-coming clutch 8 b. Typically, as the transmission 3 ages and wears, the friction coefficient μ of the clutching devices decreases. Consequently, a higher pressure is required to engage the on-coming clutch 8 b. The friction coefficient can thus be used to diagnose and predict a behaviour of the clutching devices 8 a, 8 b. The corresponding friction coefficient may be determined by using the following equation:

μ_(i)=τ_(i)/[(p _(i) −p _(i,kiss))R _(i) N _(i) A _(i)]

where τ_(i) is the amount of torque determined at the corresponding time point t1 or t2, p_(i) is the engagement pressure of the corresponding clutching device 8 a, 8 b, p_(i,kiss) is the relief pressure of the corresponding clutching device 8 a, 8 b, R_(i) is a friction plate radius, A_(i) is a friction plate area, and N_(i) is the number of friction plates of the corresponding clutching device 8 a, 8 b. In the equation above, the index i refers to the corresponding clutching device: i=1 for the off-going clutching device 8 a and i=2 for the on-coming clutching device 8 b. The pressure p_(i) in the equation above may be equal to P_(i)−P _(c), wherein P_(i) is a stagnation pressure provided by a valve pressurizing the corresponding clutching device 8 a, 8 b and P _(c) is the mean dynamic pressure on the clutch piston (see above).

The engagement pressure of the on-coming clutching device may be taken from the look-up table (cf. FIG. 12). The relief pressure or kiss pressure of the on-coming clutch may be taken from a relief pressure-torque look-up table, which is updated when the on-coming clutch 8 b is disengaged. The relief pressure or kiss pressure may be also determined by a method disclosed in WO 2014/140041 A2, which is incorporated herein by reference in its entirety.

In some embodiments, the pressure inside the clutching devices 8 a, 8 b can usually be linked to an electric current or voltage provided to an actuator to actuate a valve or a solenoid or other electric means for providing pressure inside the clutching devices 8 a, 8 b. Thus, instead of a pressure-torque relationship, a current-torque relationship or a voltage-torque relationship can also be determined. The current- or voltage-torque relationship can also be stored in a corresponding look-up table. Further, a friction coefficient of the clutching devices 8 a, 8 b can be determined on the basis of the current required to actuate the electric actuator.

The determined shift parameter can be used while performing a shift of the transmission 3. Generally, the shift is performed in accordance with the method of FIG. 2, but additional features are shown in FIG. 9. Regarding the on-coming clutch 8 b, FIG. 9 shows the overlap phase 180 b, the synchronization phase 230 and the lock phase 240. Thus, the shift is performed by disengaging the off-going clutch 8 a and engaging the on-coming clutching device 8 b. During performing the shift, torque and the pressure inside the on-coming clutching device 8 b are determined.

Generally, during the overlap phase 180 the controller 6 typically increases the pressure in the on-coming clutch 8 b to a level where the clutch 8 b may starts transferring the torque coming from the torque converter 4. To this end, the controller 6 may use a feedforward pressure profile p2 with an end point p₂₂ based on the pressure-torque-relationship of the on-coming clutch 8 b.

The on-coming clutch 8 b is generally in slip during this phase 180, 180 b. However, it is conceivable that the on-coming clutch 8 b already starts sticking because the slip speed of the on-coming clutch 8 b is small. In this situation, it is preferred that the controller 6 hold the on-coming clutch 8 b closed and increase the pressure rapidly to avoid that the clutch 8 b starts slipping again. The controller 6 may then end the overlap phase 180 and start the synchronization phase 230 and the lock phase 240 for the on-coming clutch 8 b.

However, if the torque direction phase 20 was executed because the sign of slip was previously not correct and has now been corrected by the controller 6, the slip speed will typically be small already. In this case, the described condition may be ignored to proceed to complete the hand over of the torque.

The overlap phase 180 b starts at time point t0, at which the pressure p2 of the on-coming clutch has a value p₂₁. During the overlap phase 180 b, the pressure inside the on-coming clutching device 8 b is increased to a second pressure level p₂₂ (see FIGS. 10 and 11), at which the on-coming clutching device 8 b transfers all the torque through the transmission 3, i.e. from the input of the transmission 3 to the output of the transmission 3. The second pressure-level p₂₂ is based on the pressure-torque relationship of the look-up table (see FIG. 12) determined in a previous shift.

Until the pressure inside the on-coming clutching device 8 b has reached the second pressure-level p₂₂, the pressure inside the on-coming clutching device 8 b is increased by executing a feedforward control procedure (see FIG. 9). After the pressure inside the on-coming clutching device 8 b has been increased to the second pressure-level p₂₂, the pressure inside the on-coming clutching device 8 b can be further increased by executing a feedback control procedure using the pressure as control variable, the slip speed n₂ as a process variable and a reference slip speed profile as a process variable. The feedback control procedure during the synchronization phase 230 allows the pressure value to be robust. After the synchronization 230 phase has been completed, the pressure inside the on-coming clutching device 8 b is increased with an open loop control procedure in the lock phase 240. The reference slip speed profiles mentioned in this disclosure can be based e.g. on the friction coefficient of the corresponding clutching device 8 a, 8 b and/or the pressure-torque relationship of the look-up table of the corresponding clutching device 8 a, 8 b. As such, the reference slip speed profile can be actively updated after each shift. The reference slip speed profiles can be e.g. updated by executing a feedforward control procedures. The reference slip speed profiled may be used e.g. during the synchronization phases 23, 230 and the drop phases 21, cf. FIGS. 2 and 9.

Additionally, during the overlap phase 180 a the pressure inside the off-going clutching device 8 a is decreased from a pressure value p₁₁ to a first pressure-level p₁₂ at which the off-going clutching device 8 a transfers no torque from the input to the output of the transmission 3 anymore. The first pressure-level p₁₂ is based on the relief pressure-torque relationship of the off-going clutch 8 a determined in a previous shift. Until the pressure inside the off-going clutching device 8 b has reached the first pressure-level p₁₂, the pressure inside the off-going clutching device 8 a can be decreased by executing a feedforward control procedure or a feedback control procedure (see. FIGS. 2 and 9).

FIG. 11 shows a pressure-time diagram including the pressure profiles p1 and p2 which is similar to the pressure-time diagram shown in FIG. 10. Furthermore, a rotational speed-time diagram is shown in FIG. 11. The rotational speed-time diagram includes the slip speed n₁ of the off-going clutching device 8 b (also referred to as slip speed opening clutch), the slip speed n₂ of the on-coming clutching device 8 a (also referred to as slip speed closing clutch), the output speed n_(out) of the transmission 3 (see also FIG. 1), the rotational speed n_(tur) of the torque converter (see also FIG. 1), and the rotational speed n_(drum) of the drum shaft 7, the engine speed n_(e) (see also FIG. 1). As indicated in FIG. 11, the overlap phase 180 a, 180 b ends at t1=t2. Further, the prefilling phase is indicated by the prefill pressure bump p₂₀ of the pressure signal p2.

Furthermore, a method for monitoring a state of the transmission 3 is proposed using the friction coefficients μ₁ and/or μ₂ mentioned above. First, the friction coefficients μ₁ and/or μ₂ are determined. The friction coefficients μ₁ and/or μ₂ are compared with predetermined thresholds. The compared values can be output by the controller 6. A history of stored values of the friction coefficients can also be output by the controller 6. If the friction coefficients μ₁ and/or μ₂ fall below or exceed the predetermined thresholds this may indicate that the corresponding clutching device 8 a, 8 b needs to be replaced. In this case, an alert can be output by the controller 6 to an alert device (not shown), which is configured to output a vibrating, audible or visual alert. This method is particularly useful for maintenance purposes.

The controller 6 is configured to carry out at least one of the methods described above. In a preferred embodiment, the transmission controller 6 is configured to carry out all the methods outlined above. 

1. A method for determining at least one shift parameter of a vehicle transmission, the vehicle transmission comprising a first clutching device and a first speed ratio; a second clutching device and a second speed ratio; an input; and an output, wherein the input and the output of the vehicle transmission are connectable by engaging the first clutching device or the second clutching device, the method comprising: performing a shift by disengaging the first clutching device and/or engaging the second clutching device, wherein the first clutching device stops transferring torque through the vehicle transmission at a first time point, wherein the second clutching device starts transferring torque through the vehicle transmission at a second time point; and determining the at least one shift parameter at the first time point and/or at the second time point.
 2. The method according to claim 1, further comprising: determining a rotational speed of a second input of the second clutching device; determining a rotational speed of a second output of the second clutching device; determining a second slip speed of the second clutching device while performing the shift, wherein the second slip speed is a difference between the rotational speed of the second input and the second output; determining whether the second slip speed falls below or exceeds a second predetermined threshold, the second time point being the time point when the second slip speed below or exceeds the second predetermined threshold; and determining the at least one shift parameter when the second slip speed falls below or exceeds the second predetermined threshold.
 3. The method according to claim 1, further comprising: determining a rotational speed of a first input of the first clutching device; determining a rotational speed of a first output of the first clutching device; determining a first slip speed of the first clutching device while performing the shift, wherein the first slip speed is a difference between the rotational speed of the first input and the first output; determining whether the first slip speed falls below or exceeds a predetermined threshold, the first time point being the time point when the first slip speed falls below or exceeds a first predetermined threshold; and determining the shift parameter when the first slip speed falls below or exceeds the first predetermined threshold.
 4. The method according to claim 1 wherein an amount of torque at the input of the vehicle transmission is determined at the first time point and/or the second time point.
 5. The method according to claim 1, wherein a pressure inside the first clutching device and/or a pressure inside the second clutching device are/is determined at the first time point and/or the second time point.
 6. The method according to claim 4, wherein the at least one shift parameter is a first pressure-torque relationship determined at the first time point and/or a second pressure-torque relationship determined at the second time point, wherein the first pressure-torque relationship is stored in a first look up table and the second pressure-torque relationship is stored in a second look-up table.
 7. The method according to claim 6, wherein the at least one shift parameter is a second friction coefficient μ₂ of the second clutching device, wherein the second friction coefficient μ₂ is based on the pressure-torque relationship and geometry of the second clutching device and/or wherein the shift parameter is a first friction coefficient μ₁ of the first clutching device, wherein the first friction coefficient μ₁ is based on the pressure-torque relationship and geometry of the first clutching device.
 8. The method according to claim 7, wherein a corresponding friction coefficient μ_(i) is determined using the following equation: μ_(i)=τ_(i)/[(p _(i) −p _(i,kiss))R _(i) N _(i) A _(i)], where τ_(i) is the amount of torque determined at the corresponding time point, p_(i) is the engagement pressure of the corresponding clutching device, p_(i,kiss) is the relief pressure of the corresponding clutching device, R_(i) is a friction plate radius, A_(i) is a friction plate area, and N_(i) is the number of friction plates of the corresponding clutching device.
 9. The method for performing a shift of a vehicle transmission of claim 6, comprising: performing a shift by disengaging the first clutching device and/or engaging a second clutching device; determining, when performing the shift, torque and the pressure inside the second clutching device; and increasing the pressure inside the second clutching device to a second pressure-level at which the second clutching device starts transferring torque through the transmission, wherein the second pressure-level is based on the pressure-torque relationship of the second look up table determined in a previous shift.
 10. The method for performing a shift of a vehicle transmission of claim 8, comprising: performing a shift by disengaging the first clutching device and/or engaging the second clutching device; determining, when performing the shift, the torque and the engagement pressure inside the first clutching device; and decreasing the pressure inside the first clutching device to a first pressure-level at which the first clutching device stops transferring torque through the transmission, wherein the first pressure-level is based on the pressure-torque relationship of the first look up table determined in a previous shift.
 11. The method of claim 5, wherein the pressure is determined by measuring the pressure in the corresponding clutching device by a pressure sensor or by using a pressure set point of the corresponding clutching device.
 12. The method of claim 9, wherein the pressure inside the second clutching device is increased to the second pressure-level by executing a feedforward control procedure and/or after the pressure inside the second clutching device has been increased to the second pressure-level, the pressure inside the second clutching device is further increased by executing a feedback control procedure using the pressure inside the second clutching device as control variable, a second slip speed as a process variable and a reference slip speed profile as a process variable.
 13. The method for monitoring a state of a transmission of claim 7, further comprising: determining the first friction coefficient μ₁ and/or the second friction coefficient μ₂ in accordance with claim 7; and comparing the first friction coefficient μ₁ and/or the second friction coefficient μ₂ with a predetermined threshold.
 14. The method according to claim 13, further comprising: outputting an alert if the first friction coefficient μ₁ and/or the second friction coefficient μ₂ fall below or exceed the predetermined threshold.
 15. The method according to claim 1, wherein the first clutching device and/or the second clutching device are hydraulic clutching devices, preferably wet plate clutching devices.
 16. The method according to claim 15, wherein a hydraulic pressure inside the first clutching device and/or a hydraulic pressure inside the second clutching device are/is determined at the first time point and/or the second time point.
 17. The method according to claim 15, wherein the first clutching device is emptied using a predetermined first pressure profile and/or the second clutching device is filled using a predetermined second pressure profile.
 18. The method according to claim 1, wherein the shift parameter is a first friction coefficient μ₁ of the first clutching device and/or the shift parameter is a second friction coefficient μ₂ of the second clutching device.
 19. A transmission controller for controlling a vehicle transmission, wherein the transmission controller is configured to carry out the method according to claim
 1. 